High efficiency grid modulator



April 7, 1942. D. PoLLAcK 2,279,056

HIGH EFFICIENCY GIRIDMODULATOR Filed Ju-'ly 25, 1939 2 Sheets-Sheet l2 nventor Patented Apr. 7, 1942 `rric nien EFFIcrENoY Gmo MoDULA'roR Dale Pollack, Haddon Heights, N. J., assignor to VYRadio Corporation of America, a corporation of Delaware Application Juiy 25, 1939, Serial No. 286,309

Claims.

This invention relates to modulating systems, and more particularly `to a method of and means for improving the operating eiiiciency of a modulator tube.

In high power transmitters the efficiency of operation of vthe modulator and amplifier stages is an important consideration. In gridmodulated power amplifiers for amplitude modulated waves, for example, the unmodulated carrier amplitude is equal to one-half the peak amplitude of the fully modulated carrier. Since the average modulation is far below the maximum, the all day efficiency of the system is based on tube performance which averages, say, one Vhalf its rated output, and consequently the all day eiiiciency is about one-half that which could be obtained if the amplifier were operated continuously at its rated output. In practice, Ythe all day efficiency rarely exceeds 3,0 percent.

A grid modulation system which increases the operating eiiiciency of the power amplifier tubes to a certain extent is described in an article by Terman and Woodyard, beginning at page- 929 of the August, 1938, Proceedings of the Institute of Radio Engineers. The system therein described is illustrated by Fig. 1, and includes a pair of radio frequency amplifier tubes,` carrier and peak which are connected to a load L, the connection from the carrier amplifier tube including an impedance inverter Z. The carrier tube is operated at maximum efficiency and, in the absence of modulation, Vdelivers a maximum radio frequency current equal to the amplitude of the vunmoolulated carrier. Grid modulation is used to decrease the output of the carrier amplifier for downward modulation.

The peak amplifier is biased so that it remains inactive until upward modulation begins. For upward modulation, the peak tube begins to deliver power to the load. At 100 percent upward modulation the two amplifier tubes are delivering equal power at their rated capacities, and consequently at high eii'iciency.

It will be noted that the audio modulating voltage V is applied to the grids of both the carrier and peak amplifier tubesv by means of a transformer Tl Since the carrier tube is operating near plate voltage saturation at carrier level, its output voltage is not increased substantially during upward modulation notwithstanding the fact that a large positive potential is impressed on its grid. The grid of the peak tube is also driven positive during upward modulation. The impedance of a negatively biased grid is extremely high, but the impedance of' a positively biased grid is very low. As a result the load into which the audio tube Mi is working varies between wide extremes during modulation.

A further diii'iculty is encountered in the proposed system when an attemptis made vto operate the system with degenerativeY feedback. Audio degenerative feedback can only be used successfully when the relative phases of the input and output currents are constant for all operating frequencies. That is, if the audiorfrequency coupling transformer T1 has a high leakage reactance the low and high frequency currents will be shifted in phase more than `the midrange frequencies, and as a result the feedback voltage derived from its output will notV be in proper phase to produce equal degenerative effects at all frequencies. A transformer having a minimum leakage reactance is a 1:1 ratio, closely coupled transformer. Such a transformer, however, has a high secondary impedance. However, because of the high current required by the radio frequency amplifier grids during upward modulation, the impedance of the audio frequencyxsource must be kept low. The high grid current, therefore, makes necessary the use of a'tr'ansformer having a large step-down ratio while the use of feedback makes necessarya 1:1 transformer. Obviously, both requirements cannot be met at the same time with a given transformer.

Itis, therefore, a principal object of thisinvention to provide an improved modulating system in which feedback may be employed satisfactorily and which eliminates the necessity of compromising between two design considerationsv which have opposing solutions. Other objects of this invention include the provision of an improved grid modulating system; the provision of means for increasing the eiiiciency of the audio amplifier tubes when delivering energy into a load impedance of varying magnitude; and the provision of an improved modulating system uti lizing degenerative feedback.

This inventionwill be better understood from the following description when considered yin connection with the accompany drawings. Its scope is indicated by the appended claims.

Referring to the drawings, YFigure 1 is a basic modulating` system illustrative of the prior art; Figure 2 is a curve which explains one feature of this invention; Figure 3 is a modulating system in accordance with this invention; Figure 4 is an alternative modulator vcircuit; of low impedance; Figure 5 is a low impedance modulator utilizing twoforms of degenerative feedback; and Figure 6 is an alternative feedback modulator. The operation of the modulator tube will first be considered, and Ia system shown byl whichV its efficiency may be increased. Referring to Fig. 2, a family of curves is illustrated which represent the plate voltage-plate current Acharacteristic of a modulator tube for various values of grid bias.

The load yline for a modulator tube MI for example, 'connected as illustrated in Fig, 1, is rep resented by the curve A-B-C of Fig. 2, and is horizontal from A to B. As is well known, the slope of the load line is a measure of the reciprocal of the load impedance. This region, therefore, corresponds to the range over which the grids of the radio frequency carrier and pea amplifiers of Fig. 1 are most negative with respect to their cathodes and present a high impedance to the modulator tube MAI. A consideration of the circuit of Fig. 1 shows that an upaavaose .1T 5 A lsimilar to that illustrated in Fig. 1 with regard to the connection of the carrier and peak radio frequency amplifiers to the load L. A couward modulation of the modulator plate current produces a downward modulation of the amplier plate current and thus of the output. That is, vthe normal or xed negative biases of the two amplifier tubes are increased l with increasing modulator plate current and decreased with decreasing modulator plate current. The load line curves sharply downward from B toC. This region represents the range over which the grids of the radio frequency carrier and peak amplifiers of Fig. 1 are becoming less negative, that is, more positive and present a low impedance to the modulator tubes.A As indicated, minimum allow the tube to operate over the full swing which is necessary to produce 100 percent modulation upward.` For a' pair of type 898 tubes, for example, used-as radio frequency amplifiers in ak. w. transmitter, the required static modulator plate current is about 0.8 ampere per tube,` and the steady modulator dissipation for the unmodulated condition is about 5 k. w.

In accordance with'this invention, the polarity of the modulatingtransformer Tl is reversed so-that the static or carrier condition is represented by a low modulator plate current, and upward modulation of the output is made to correspond to an increase'in the modulator plate current. The dynamiccharacteristic of the modulator then `has the shape shown by the load line D-E-F, Fig. 2. The downward modulation and carrier condition then rfalls on the horizontal portion of the line D-.E, and the modulator plate current 4is at a very low value, Ips. --I-Iowever, because the load impedance is high, small changes in plate current produce a large change in voltage kacross the load. Upward modulationr corresponds to the increasing `load portion of the load line E-F. Since percent modulation is reached infrequently the modulator plate current is normally well below the steady state value Ips of the first condition, so that a considerable reduction in the modulator' plate dissipation is effected.

The load line D-E--F is for operation with a purely resistive load. When the modulator load is reactive, as it is at high Vand low audio frequencies, the load line becomes a distorted ellipse, as shown in the dotted curve of Fig. 2. This curve must not touch they zero plate current axis or there will be considerable distortion in the modulated carrier. It is thelast named requirement that dictates the minimummodulator static plate current Ips. If--the-,load were purely pling transformer T2 having separate secondaries S1 and S2 is provided, however, and, as before, a single modulator tube M is illustrated. The connections to the secondary windings are reversed so that the initial or fixed negative biases of the two amplifier tubes are decreased with increasing modulator plate current and increased with decreasing modulator plate current. In this case, when the modulator plate current increases from the no signal value, the amplifier grids become more positive. thus decreasing the load impedance, showing that the modulator is operating on the load line D-E-F. In the system suggested AbyfIerman, referred to above, an increase in modulator'plate current caused the amplifier grids tobecome less positive, thus increasing the load impedance. l

The circuit illustrated in Fig. 3 is an improvement over the prior art in that it greatly increases the modulator efficiency, but it does not solve the diniculty of designing a modulation transformer. This design conflict was mentioned briefly above. In accordance with a modification of this invention a modulator circuit is now proposed ,which not only has ther increased modulator eficiency of the system `just described but which also has-the further advantage of-having a low output impedance.` Variations in the grid input impedance of aY radio frequency amplifier seriously distort the A.modulation 'characteristic whenthe modulator impedance is high, because there is a large voltage drop in the modulating circuit which includes the secondary Winding of the modulation transformer. An appreciation of the magnitude of thisdistortion in a high power transmitter may be had when it is realized that the grid current of a large tube may be as high as'3 amperes. Even if the impedance of the circuit through which the grid current must flow is kept as low as, say, 100 ohms, there will be a voltage dropof 300 volts, which, ofcourse, is deducted from the voltage which should be applied tothe amplifier grid.

a positive D.C. potential.

' amplifiers.

One circuit for` decreasing the modulator impedance is illustrated in Fig. 4, to which reference is now made. M3 is a modulator tube, the input to which is coupled to a source of audio voltage V. The tube is operated as a cathode follower amplifier having its anode at ground potential at the audio frequency but which is at The modulator cathode is connecteddirectly to the grid of one of the amplifier tubes, for example, the carrier amplifier of Figs. 1 and 3. It is capacitively coupled to the grid of the other amplifier by a capacitor CI the purpose of which is to permit separate bias potentials to be applied to the two 'I'hus the peak tube is biased by a battery K connected to the grid through a resistor N. It is tobe understood, however, that under certainv conditions the two tubes may be operated at the same bias voltage, in which case the coupling capacitor CI and separate bias supply will not be required. The cathode is also connected to ground through a load reactor or chokeCH and a biasing battery BAT which ,sup-

plies grid bias to one of the modulated amplifier grids and also assists in maintainingthe required anode-cathode potential of the modulator M3. Alternatively, another modulator, similarly connected, may be used to modulate the peak amplifier, the inputs of the two modulators being connected in parallel to the source of audio voltage.

As is well known, the impedance across the cathode load of a cathode follower tube is quite low. The losses in the tube are likewise low, since a consideration of the circuit shows that the tube is operating on the inverted DEF characteristic of Fig. 2. That is, as the modulator grid becomes positive, the cathode tends to follow it. It too, therefore, becomeslmore positive, thus applying a more positive voltage to the amplifier grids, causing the load impedance to decrease and the carrier amplitude to increase.

There is a saving in the elimination of the modulation transformer, for a transformer is usually more costly than a simple reactor, and the problem of obtaining satisfactory degenerative feedback is also made easier because the -phase shift at the extreme frequency ranges is reduced.

The modulation circuit illustrated inv Fig. 4 has one disadvantage, and that is that it has substantially no gain as an amplifier. That is, if

1000 Volts modulating voltage is required for the radio frequency amplifier grids, 1000 volts must be applied to the modulator grid. The addition of regenerative feedback would increase the gain, but the instability of a regenerative amplifier more than offsets its advantages. It has been found, however, that regenerative feedback may be safely used in conjunction with degenerative feedback. Such a circuit is illustrated in Fig. 5, to which reference is now made.

An amplifier tube M4 and a cathode follower' modulator tube M5 are resistance-capacitance coupled, and the audio input voltage V is applied to the grid of the amplifier M4. The plate of the modulator M5 is grounded, as before, and

its cathode is coupled to the grids of the radio 7 frequency amplifier or amplifiers, as the case may be, which have not been illustrated again since the amplifier circuit is identical to that shown in the preceding figures.

Regenerative feedback is used on the modulator tube M5. The high potential terminal of the plate load reactor CH is coupled back to the modulator grid through a phasing network PH, if necessary. Degenerative feedback is obtained by including a resistor P in series with the modulator plate reactor CH and applying the voltage developed across the resistor to the input of the amplifier M4. As before, a phasing network PH may be included in this connecten. Since the grids of amplifier Md and modulator M5 are in phase opposition, due to the phase reversal of the amplifier tube M4, the feedback'to the modulator M5 will increase its gain and the feedback to the amplifier M4 will be degenerative sufciently to prevent instability. The regenerative feedback must, of course, exceed the degenerative feedback if amplification is to be realized. This system has all the advantages of the prior systems since the cathode follower circuit has inherently low self impedance, since the modulator efficiency is good as the tube operates on curve DEF of Fig. 2, and since substantial gain is available without instability.

In addition to increasing the gain of an amplifier, it has been found that the plate or output impedance of vsuch an amplifier is greatly decreased. The necessity for havingva low self impedance in the modulator 4tube has been discussed above. The use of feedback to decrease the normal plate impedance of a modulator is therefore proposed as a further modification of this invention. Such a system is illustrated in Fig. 6, to which reference is now made. 'y

An amplifier M4 is resistance coupled to a conventional modulator MB. The amplifier MIS is energized bythe audio input voltage V as in Fig. 5 while the plate of the modulator M6 is coupled to the radio frequency amplifier grids. The regenerative and degenerative feedback connections are made from the output of the modulator M6 to the amplier M4 and modulator M6 grids, respectively, as was done in the-system illustrated in Fig. 5. In the present instance, however, the overall feedback to the amplifier M4 is regenerative, since the output voltage is in phase -with the input voltage, while the modulator feedback is degenerative. By providing a resistive plate load R, the phasing networks illustrated in Fig. 5 may not be necessary.

There is a further advantage in the systems illustrated in Figs. 4 to v6 which has notyet been mentioned. The audio feedback systems illustrated in Figs. 5 and 6 and the elimination of the modulation transformer in Fig. 4 make possible more effective overall transmitter degenerative feedback. Overall transmitter degenerative feedback is accomplished by rectifying a voltage derived from the transmitter output and applying the rectified voltage in the proper phase to the audio input. This can, however, only beaccomplished satisfactorily where the phase shift of Vthe various audio frequency current components is uniform throughout the entire transmitter. By reducing the distortion in the audio system and by removing elements of the audio system whichV contribute to -non-uniformity it will be appreciated that the problem of overall feedback is greatly simplified.

I claim as my invention:

1. In a grid modulated amplier the combination which includes a radio frequency amplier tube having an input electrode, a modulator tube having an output electrode, means coupling said output electrode to said input electrode, said means including a transformer which is so connected that maximum current to said modulator output electrode produces maximum positive potential on said amplifier input electrode, and minimum current tosaid modulator output electrode produces 'minimum positive potential on said amplifier input electrode.

2. A modulation system which comprises a radio frequency amplifier having at least one grid electrode, a source of audio frequency modulating voltage, a modulator tube having grid, cathode and anode electrodes, means for impressing said modulating voltage between said grid and anode electrodes, means coupling said cathode electrode to the grid Iof said radio frequency amplifier, and meansfor applying oper-l ating potentials to said modulator Vand amplifier electrodes whereby the audio frequency potential of said cathode is impressed on the grid of said amplifier to vary the amplification of said amplifier.

3. A modulation system which comprises a radio frequency amplifier having at least vone electrode for varying the amplification of said amplifier in accordance with changes in its potential, means for varying the potential of said electrodeicomprising a modulator tube having input andoutput circuits, said output circuit being coupled to said electrode, means for applying a modulating voltage to said input circuit, means for also applying to said input circuit a regenerative voltage proportional to the voltage developed in said output circuit and in phase with said modulating voltage, and means for also applyingl to said input circuit a degenerative voltage proportional to the voltage developed in-said output circuit, in phase opposition to said-modu-v lating voltage, and of a magnitude to stabilize the operation,.,whereby increased gain without oscillations isrproduced. f

4. In a gridmodulated amplifier the combination which includes ay first radio frequencyV amplifler having input and output electrodes, a source of unmodulated radio frequency voltage, means for applying said radio frequency voltage to said input electrode, a utilization device, an impedance inverter coupling said utilization device to said output electrode, `an auxiliary amplifier having input and output electrodes, said input electrode being coupled to said source of unmodulated radio frequency and said output electrode being coupled to said utilization device, a modulating tube, means for modulating the plate current of said tube in accordance with signal representing voltages, and means coupling the input electrodes of said first and `auxiliary amplifiers to said modulating tube for applying modulating potentials to said input electrodes which increase in a positive direction as a function of an increase in said plate current. Y

5. In agrid modulated amplifier the combination which includes la first `radio frequency ampliiier having input and output electrodes, a source of unmodulated radio frequency voltage, means for applying said radio frequency voltage to said input electrode, a utilization device, an impedance inverter coupling said utilization device to said output electrode, an auxiliary amplifier having input and output electrodes, said input electrode being coupled to said source of unmodulated radio frequency and said output electrode being coupled to saidutilization device, a modulating tube, means for modulating the plate current of said tube in accordance with signal representing voltages, means for applying negative bias voltages to the input electrodes of said radio frequency and auxiliary amplifiers, and means coupling both of said Vinput electrodes to said `modulating tube for decreasing said bias voltage with increasing plate current of said modulating tube and for increasing said bias voltage Withdecreasing plate current of said modulating tube. q

6. In a modulation systemfhaving means including a first amplifier tube for applying carrier frequency currents to an output device, means including a second amplifier tube for applying in phase currents to `said output device, and means including a modulator tube for applying' modulating voltages to said amplifier tubes, the method of operation which includes the steps of applying` fixed biasing potentials to said amplifier-tubes, decreasing said biasing potentials in accordance with increasing currents in said modulator tube and increasing said biasing potentials in accordance with decreasing currents in said modulator tube.

7. In a modulation system having means including a first amplifier tube for applying lcarrier frequency currents to an output device, means including a second amplifier tube for applying in phase currents to said output device'and means including a modulator tube for applying modulating voltages to said amplifier tubes, the method of operation which includes the steps -of `applying an initial biasing potential to said first amplifier tube to cause said tube to applyvan-unmodulated carrier of predetermined amplitude to said output device at high efficiency, vapplying an initial negative biasing potential to said second amplifier tube to cause said tube to be normally inactive, decreasing the biasing potential of said second amplifier in accordance with increasing currents in said modulator tube to thereby increase the carrier amplitude applied to said output device above said predetermined value, and increasing the negative biasing potential applied to said first amplifier in accordance with decreasing currents in said modulator tube to thereby decrease the carrier amplitude applied to said output device below said predetermined value.

8. A modulation system which comprises a radio frequency amplifier having at least one grid electrode, a source of audio frequency modulating voltage, a modulator tube having grid, cathode and anode electrodes, means for impressing said modulating voltage between said grid and anode electrodes, means coupling said cathode electrode to the grid of said radio frequency amplifier, means for impressing a voltage proportional to the voltage of said cathode electrode and in phase therewith between said grid and anode electrodes, and means for impressing a voltage proportional to the voltage vof said cathode and in phase opposition thereto `between said grid and anode electrodes. 1

9. Amodulation system which comprises a radio frequency amplifier having at least one grid electrodeya source of audio frequencyvmodulating voltage, a modulator tube having grid, cathode and anode electrodes, means `for impressing said modulating voltage between said grid and `anode electrodes, means coupling said cathode electrode to the grid of said radiofrequency amplifier, and'means for impressing degenerative and regenerative feedback voltages betweensaid grid and anode electrodes in such proportions that the gain of the amplifier is increased within a stable range of operation.

10. A modulation system which comprises a radio frequency amplifier having at least one grid electrode, a source of i audio frequency modulating voltage, a modulator tube having grid, cathode and anode electrodes, means `for impressing said modulating voltage between said grid and anode electrodes, means coupling said cathode electrode to the grid of said radio'frey quency amplifier, means for impressing a regenerative feedback voltage between said` grid and anode electrodes to increase the amplification of said modulator tube by a certain amount, and means for impressinga degenerativefeedback voltage between said grid and anode electrodes to decrease the amplification of said modulator by an amount less than said `certain amount, whereby increased and stabilized overall ampliiication in said modulator is produced.

i DALE POLLACK. 

